1. Field of the Invention
The present invention generally relates to aircraft display and control systems, and more particularly, relates to a system providing a uniquely integrated display of aircraft performance and for controlling the flight path of the aircraft in which the thrust is varied to maintain the aircraft on a desired glide path, and the pitch of the aircraft is varied to maintain a desired angle of attack.
2. Description of the Prior Art
In normal flight, an aircraft's motion is controlled by its heading, thrust and angle of attack. Since air speed is a direct function of an aircraft;'s angle of attack, the angle of attack is usually controlled by a reference to the aircraft's air speed. Pitch and thrust, however, have primary and secondary effects upon an aircraft's flight performance. The primary effect of a change in pitch is that it causes a directly proportional change in the angle of attack because of the relationship between the aircraft's angle of attack, its pitch, and its flight path angle. The secondary effect of a change in pitch is that it causes a change in the thrust requirement to maintain a steady state flight condition. This secondary effect occurs because of the relationship between the angle of attack and the drag. Moreover, the secondary effect of a pitch change is highly variable and can be either positive or negative. The primary effect of a change in thrust is that it causes a change in the aircraft's flight path angle which is, in turn, dependent upon the relationship between the flight path angle, drag and thrust. The secondary effect of a change in thrust is that it causes a change in the angle of attack because of the relationship between the angle of attack, flight path angle and pitch.
Prior art manual and automatic control system which use computed command signals for control of the aircraft on approach have used a method of control whereby pitch is used to control the aircraft's position on a glide path with the resultant effect that the angle of attack was corrected with thrust corrections. This method is quite useful for cruise flights where strong speed stability exists; however, the use of this method during an approach operation has serious faults since there is a time lag before the effects of a control are observed. For example, if an aircraft is going below the glide path, additional thrust is needed to increase the flight path angle, but the prior art systems have instead changed the pitch to cause the aircraft to fly to the glide path. This causes an increase in angle of attack which is indicated by a decrease in air speed. This change in angle of attack or air speed is then noted and a thrust correction is applied which changes the flight path angle, resulting in a change to the angle of attack. When the aircraft returns to the glide path, the pitch is again changed and the thrust is also varied after the results of the pitch change are evident.
In my prior U.S. Pat., No. 3,586,268, I disclose an improved flight control system in which, in the approach mode, the angle of attack of the aircraft is controlled entirely by pitch command signals and the position on a glide path is controlled entirely by thrust command signals. Thus, my earlier system implements the principle that the thrust level, rather than the pitch, should be used to correct for glide path displacements, and the pitch should be used to control the angle of attack or air speed. The actual method employed is first, to reduce the rate of displacement of the aircraft from the glide path to zero by thrust control, and second, by further thrust control to gradually return the aircraft to the glide path. The magnitude of the corrective thrust which must be applied for a given angular displacement from the glide path is proportional to the distance the aircraft is from the touch-down point. Hence, corrective thrust for a given angular displacement from the glide path is applied at a greater magnitude when the aircraft is at a greater distance from touch-down. At all times during the thrust corrections, pitch command signals enable the pilot or auto-pilot to control the aircraft with reference to a desired angle of attack. Separate pitch and thrust command displays are provided to facilitate manual approach operation by the pilot.
Existing flight instruments display the horizon reference relative to the aircraft's pitch. The problem with present aircraft instrumentation is that pitch or attitude is not directly useable by the pilot for control of the aircraft. The pitch information must be integrated with other information such as sink rate and air speed for the pilot to determine exactly what is happening to his aircraft. This problem has been recognized in the prior art, and there have been several attempts to provide integrated flight instrumentation. One such example is the patent to J. O. Nesbitt, Ser. No. 2,941,400 which discloses a flight control indicator having a fixed reference aircraft mark, a rate of change in altitude bar and an angle of attack bar. The Nesbitt instrument displays rate of climb or descent as the primary reference, and while Nesbitt does provide an angle of attack display, this display is not in terms of an angular function the pilot can readily read. Instead, the Nesbitt display causes the angle of attack indicator to go out of view at low angle of attack (high speed) and to descend toward the reference bar at high angles of attack (low speed). When the angle of attack indicator touches the reference bar in the Nesbitt display, the aircraft is in a stall condition (maximum angle of attack for all practical purposes). Thus, Nesbitt's angle of attack bar is really a stall warning device and does not indicate the amount of angle of attack. Furthermore, Nesbitt's choice of having the zero rate of climb reference fixed in terms of the reference bar, causes an increasing sink rate to be read as an increasing upward number, and vice versa, making interpretation of the display rather difficult.
Notwithstanding the limitations just noted, the Nesbitt flight control indicator is admirable for its relative simplicity. The recent trend in the industry is to provide very sophisticated and complex integrated flight instrumentation. Because of their sophistication and the amount of information to be displayed, such flight instruments typically are implemented with cathode ray tube (CRT) displays. Typical of such instrumentation is the U.S. Pat. No. 3,668,622 to James R. Gannett et al. Typically, in such sophisticated flight instrumentation, a number of flight parameters and commands are displayed. For example, in the Gannett et al. system, flight path angle is displayed, but it is not correlated to the other functions. The result is a complicated maze of information which is very difficult for the pilot to interpret.